Earlier this month, the Washington Post reported that sources said Brown initially went to the hospital because of arthritis problems, and there, doctors found a blood clot. Brown had an operation to have the clot removed, but he then developed pneumonia, the Washington Post reported.

Pneumonia occurs when there is some sort of infection -- whether it be bacterial or viral, or caused by fungi or parasites -- that leads to inflammation of the lungs. The most common bacterium that causes pneumonia is Streptococcus pneumonia, when acquired outside of the hospital.

In 2009, 1.1 million hospital discharges were for pneumonia, according to the Centers for Disease Control and Statistics. There were 50,774 deaths in the U.S. from the condition -- which comes out to 16.5 deaths for every 100,000 people.

Pneumonia and influenza are, together, considered the No. 8 leading cause of death in America, according to the American Lung Association, with more deaths from pneumonia than from influenza.

Pneumonia does not usually result in complications for younger people if they take the right medicines (like antibiotics if the pneumonia was caused by a bacteria), according to the Mayo Clinic. But pneumonia can lead to dangerous complications, particularly for people older than age 65, as well as people who have lung disease or heart failure, or people who smoke.

One complication is when fluid accumulates in the membranes that surround the lungs, which can lead to infection. Another is bacteria in the bloodstream, which can lead to shock, organ failure and possibly even death, the Mayo Clinic reported.

Acute respiratory distress syndrome -- which is when it's hard to breathe and for your body to receive oxygen -- and lung abscesses -- when a pus-filled cavity grows in the part of the body where the pneumonia is -- are also complications of the condition, according to the Mayo Clinic.

The condition can develop when the causes of pneumonia -- like bacteria and viruses -- go into the lungs from their normal living spaces, such as your sinuses or mouth, according to the National Institutes of Health. It's also possible to breathe these pathogens into the lungs. Another form is aspiration pneumonia, which occurs when you inhale food, liquid or some other fluid into the lungs.

Pneumonia can also be acquired while in the hospital, and is more common in patients who are on a respirator, according to the A.D.A.M. Medical Encyclopedia. Hospital-acquired pneumonia is usually considered to be quite dangerous. Patients who have a lowered immune system, have some sort of chronic lung disease, are older, are alcoholic, who have just had surgery, or who may have problems swallowing are also at a higher risk of acquiring pneumonia in the hospital.

Symptoms of pneumonia include fatigue, cough, fever, chills and sweating, chest and muscle pain, headache and shaking chills, according to the Mayo Clinic. The condition is diagnosed with chest X-rays (where doctors can see where in the body the infection is located), a physical exam and/or a blood or mucus test.